High-data-rate communication link using multiple lower rate modems

ABSTRACT

A method for communication includes receiving a composite signal, which carries data at a first data rate and includes multiple sub-signals that are interleaved in a time domain and are separated by boundary indicators. The received composite signal is demultiplexed into the sub-signals by automatically detecting the boundary indicators between the sub-signals in the composite signal. The sub-signals are demodulated using multiple respective demodulators operating at second data rates that are lower than the first data rate so as to generate respective output data streams. The output data streams are combined so as to reconstruct the data at the first data rate.

FIELD OF THE INVENTION

The present invention relates generally to communication links, andparticularly to methods and systems for transmitting and receiving athigh data rates using multiple modems.

BACKGROUND OF THE INVENTION

Several methods and systems are known in the art for transmitting andreceiving at a high data rate using multiple lower data rate modems. Forexample, PCT Publication WO 2006/097735, which is incorporated herein byreference, describes an apparatus and method for processing ahigh-data-rate signal. The apparatus comprises a plurality of dataprocessing units and switching means to repeatedly switch a receivedhigh-data-rate signal between inputs of the plurality of data processingunits on a time division basis. Each of the data processing units isoperable to process the signal at a data rate lower than the high datarate, and the apparatus further comprises combining means operable toswitch between outputs of the plurality of data processing units to forma processed high-data-rate signal. A receiver apparatus is alsodisclosed, operable to cooperate with the apparatus to receive aprocessed signal and to undo each of the processing steps performed bythe apparatus to convert the processed signal into an original highfirst data rate signal.

As another example, U.S. Pat. No. 5,809,070, whose disclosure isincorporated herein by reference, describes methods and apparatus forproviding high-speed inter-computer data transmission using multiple lowspeed communication links. At the transmitting site, a high-speed datastream is split into multiple low-speed data streams and multiplexedonto low-speed links. The receiver demultiplexes, buffers andsynchronizes the multiple low-speed data streams to recreate thehigh-speed data stream.

U.S. Pat. No. 6,647,059, whose disclosure is incorporated herein byreference, describes a low-cost data communication system using modems.In the proposed system and method, an incoming binary data stream issplit into several parallel sub-streams by an encoding modem. Theparallel sub-streams are mapped into a set of unique orthogonalshort-codes and then modulated by a bank of modulators. Thereafter, themodulated data is combined and transmitted through a wired communicationchannel, such as a cable or optical fiber channel. A decoding modem canreceive encoding information from the encoding modem and appropriatelydecode the transmitted information.

SUMMARY OF THE INVENTION

There is therefore provided, in accordance with an embodiment of thepresent invention, a method for communication, including:

receiving a composite signal, which carries data at a first data rateand includes multiple sub-signals that are interleaved in a time domainand are separated by boundary indicators;

demultiplexing the received composite signal into the sub-signals byautomatically detecting the boundary indicators between the sub-signalsin the composite signal; and

demodulating the sub-signals using multiple respective demodulatorsoperating at second data rates that are lower than the first data rateso as to generate respective output data streams, and combining theoutput data streams so as to reconstruct the data.

In some embodiments, the composite signal originates from a singletransmitter. In an embodiment, the demodulators include burstdemodulators. In another embodiment, the demodulators include streamdemodulators.

In yet another embodiment, the boundary indicators include silentperiods. Automatically detecting the boundary indicators may includecalculating an instantaneous power of the received composite signal anddetecting the boundary indicators responsively to the calculatedinstantaneous power.

In still another embodiment, the boundary indicators include sequencesof known symbols. Automatically detecting the boundary indicators mayinclude calculating correlations between samples of the receivedcomposite signal and between the sequences of the known symbols.Additionally or alternatively, automatically detecting the boundaryindicators includes accepting one or more correlation indications fromone or more of the multiple demodulators, which indicate a correlationbetween the sub-signals demodulated by the respective demodulators andbetween the sequences of the known symbols.

In a disclosed embodiment, the boundary indicators include sequences ofknown samples having the first data rate, and automatically detectingthe boundary indicators includes calculating correlations betweensamples of the received composite signal and between the sequences ofthe known samples. In another embodiment, the boundary indicatorsinclude sequences of known bits.

In yet another embodiment, demodulating the sub-signals includesrecovering a timing of at least one sub-signal by the respectivedemodulator, and receiving the composite signal includes digitizing thecomposite signal using an analog-to-digital converter (ADC) andadjusting a sampling clock of the ADC based on the recovered timing.

In still another embodiment, receiving the composite signal includesdigitally recovering a timing of the composite signal. In an embodiment,recovering the timing of the composite signal includes estimating atiming offset of at least one sub-signal by the respective demodulator,and correcting a corresponding timing offset in the composite signalusing digital interpolation responsively to the calculated timing offsetof the at least one sub-signal. In another embodiment, the compositesignal includes a sequence of known samples, and recovering the timingof the composite signal includes estimating a timing offset in thecomposite signal by processing the known samples, and correcting thetiming offset in the composite signal by applying digital interpolationto the composite signal.

In some embodiments, at least one of the demodulators includes anadaptive receiver loop used for demodulating the respective sub-signal,and demodulating the sub-signals includes compensating for an error ofthe adaptive loop accumulated between successive time intervals of thesub-signal. In an embodiment, the adaptive loop includes a phaserecovery loop, and compensating for the error includes compensating fora phase error accumulated between the successive time intervals of thesub-signal. Additionally or alternatively, the adaptive loop includes atiming recovery loop, and compensating for the error includescompensating for a timing error accumulated between the successive timeintervals of the sub-signal. In a disclosed embodiment, at least two ofthe second data rates are different from one another.

There is additionally provided, in accordance with an embodiment of thepresent invention, a method for communication, including:

demultiplexing an input data stream having a first data rate intomultiple sub-streams having respective second data rates that are lowerthan the first data rate;

modulating the sub-streams using respective multiple modulators, whichoperate at the second data rates;

interleaving the modulated sub-streams in a time-domain to produce amodulated composite signal;

inserting boundary indicators between the modulated sub-streams in thecomposite signal; and

transmitting the composite signal over a communication link.

In some embodiments, the boundary indicators include at least oneindicator type selected from a group of indicator types consisting ofsilent periods, sequences of known symbols, sequences of known samplesand sequences of known bits.

In another embodiment, modulating the sub-streams includes causing aspectral response of the composite signal to comply with a givenspectral mask by causing the spectral response of each of thesub-streams to comply with a spectrally scaled-down replica of thespectral mask. Causing the spectral response of the composite signal tocomply with the given spectral mask may include separating thesub-signals in the time domain with separators, which include at leastone separator type selected from a group of types consisting of a guardinterval and a symmetric sequence of known symbols.

There is also provided, in accordance with an embodiment of the presentinvention, a receiver, including:

a front-end, which is arranged to receive a composite signal, whichcarries data at a first data rate and includes multiple sub-signals thatare interleaved in a time domain and are separated by boundaryindicators;

a separator, which is arranged to demultiplex the received compositesignal into the sub-signals by automatically detecting the boundaryindicators between the sub-signals in the composite signal;

multiple demodulators, which operate at second data rates that are lowerthan the first data rate and are arranged to demodulate the respectivesub-signals so as to generate respective output data streams; and

a multiplexer, which is configured to combine the output data streams soas to reconstruct the data.

There if further provided, in accordance with an embodiment of thepresent invention, a transmitter, including:

a demultiplexer, which is arranged to demultiplex an input data streamhaving a first data rate into multiple sub-streams having respectivesecond data rates that are lower than the first data rate;

multiple modulators, which respectively operate at the second data ratesand are arranged to modulate the sub-streams;

a combiner, which is arranged to interleave the modulated sub-streams ina time-domain to produce a modulated composite signal; and

a boundary marker, which is arranged to insert boundary indicatorsbetween the modulated sub-streams in the composite signal.

In some embodiments, the boundary marker is resident in at least oneunit selected from a group of units consisting of the demultiplexer, themodulators and the combiner.

There is additionally provided, in accordance with an embodiment of thepresent invention, a communication link, including:

a transmitter, which includes multiple modulators and is arranged todemultiplex input data having a first data rate into respectivesub-streams having second data rates that are lower than the first datarate, to modulate the sub-streams using the respective modulators thatoperate at the second data rates, to interleave the modulatedsub-streams in a time-domain to produce a modulated composite signal, toinsert boundary indicators between the modulated sub-streams in thecomposite signal and to transmit the composite signal; and

a receiver, which includes multiple demodulators that respectivelyoperate at the second data rates and is arranged to receive thecomposite signal, to demultiplex the received composite signal into thesub-signals by automatically detecting the boundary indicators, torespectively demodulate the sub-signals using the demodulators so as togenerate output data streams, and to combine the output data streams soas to reconstruct the input data.

There is also provided, in accordance with an embodiment of the presentinvention, a method for communication, including:

demultiplexing input data having a first data rate into multiplesub-streams having respective second data rates that are lower than thefirst data rate;

modulating the sub-streams using respective modulators operating at thesecond data rates;

interleaving the modulated sub-streams in a time-domain to produce amodulated composite signal;

inserting boundary indicators between the modulated sub-streams in thecomposite signal;

transmitting the composite signal over a communication channel;

receiving the composite signal transmitted over the communicationchannel;

demultiplexing the received composite signal into the sub-signals byautomatically detecting the boundary indicators;

demodulating the sub-signals using respective demodulators operating atthe second data rates so as to generate output data streams; and

combining the output data streams so as to reconstruct the input data.

The present invention will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram that schematically illustrates a communicationlink, in accordance with an embodiment of the present invention;

FIG. 2 is a diagram that schematically illustrates conversion of signalsproduced by multiple modulators into a high-data-rate composite signal,in accordance with an embodiment of the present invention;

FIG. 3 is a block diagram that schematically illustrates a receiver thatuses multiple demodulators, in accordance with an embodiment of thepresent invention; and

FIG. 4 is a flow chart that schematically illustrates a method forcommunicating at a high data rate using multiple modems, in accordancewith an embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS Overview

Embodiments of the present invention provide improved methods andsystems for sending data from a transmitter to a receiver at a high datarate using multiple modems, which operate at a lower data rate. In someembodiments, the transmitter partitions an input data stream intomultiple sub-streams. The sub-streams are respectively processed bymultiple modulators, each operating at a data rate that is lower thanthe rate of the input data stream. The transmitter interleaves theoutputs of the modulators in the time-domain to produce a modulatedcomposite signal. The outputs of the modulators, i.e. the modulatedsub-streams, are referred to herein as sub-signals.

Typically, the transmitter inserts boundary indicators between thesub-signals in the composite signal, in order to enable the receiver todetect these boundaries and differentiate between the differentsub-signals. The boundaries can be marked, for example, using silentperiods or using known bit or symbol sequences. The composite signal isthen transmitted to the receiver.

The receiver receives the composite signal and automatically detects theboundaries between the modulated sub-signals using the boundaryindicators. Based on the detected boundaries, the receiver separates thecomposite signal into the individual sub-signals. The modulatedsub-signals are processed by respective demodulators, each operating ata data rate that is lower than the data rate of the composite signal(i.e., the data rate of the input data stream). The receiver multiplexesthe outputs of the demodulators, to produce an output data stream thatreconstructs the high-data-rate input data stream.

Unlike some known methods in which the input data is partitioned intosub-streams in accordance with a fixed protocol agreed between thetransmitter and receiver, the methods and systems described hereinenable flexible partitioning of the data among the different modems,without prior coordination of the switching times between thetransmitter and receiver.

Several exemplary transmitter and receiver configurations are describedhereinbelow. In particular, different receiver mechanisms for detectingthe boundaries between sub-signals, and for synchronizing the receptionof the different sub-signals, are described. For example, the modems(i.e., modulators and demodulators) in the transmitter and receiver maycomprise burst modems or continuously-operating stream modems.

In some cases, the transmitter and receiver may use existing modemdevices and circuitry, with little or no modification. For example, somereceiver configurations described below use certain synchronizationindications provided by the individual demodulators for detecting theboundaries between sub-signals.

System Description

FIG. 1 is a block diagram that schematically illustrates a communicationlink 20, in accordance with an embodiment of the present invention. Asin the exemplary embodiment shown in FIG. 1, link 20 may comprise awireless link, such as a point-to-point microwave or millimeter wavelink. Alternatively, link 20 may comprise a wireline link, such as adata-over-cable link.

Link 20 comprises a transmitter 24, which transmits data at a high datarate to a receiver 28. For example, the link may carry aGigabit-Ethernet (GbE) connection, a Synchronous Transfer Mode 4 (STM4)link, an Optical Carrier 12 (OC12) link, or any other suitablehigh-data-rate connection.

The transmitter accepts an input data stream having a high data rate. Ademultiplexer 32 partitions the input data stream into multiplesub-streams. The sub-streams are processed in parallel by respectivemodulators 36, which operate at data rates that are lower than the datarate of the input data stream. Modulators 36 produce modulatedsub-streams, or sub-signals, at their outputs. Each modulator 36typically encodes its data sub-stream using a Forward Error Correction(FEC) code and modulates the encoded data in accordance with a certainmodulation scheme, to produce a sequence of modulated symbols.

The modulators may use any suitable modulation scheme, such asquaternary amplitude modulation (QAM) or phase-shift keying (PSK), andthe modulated symbols may be represented by any suitable number ofsamples per symbol.

In some cases, the modulators add known symbol sequences, such aspreambles, to the sub-signals. The use of such symbol sequences isdescribed further below.

The outputs of modulators 36 are buffered by buffers 40. A streamcombiner 44 interleaves the multiple streams of modulated samples into asingle composite modulated signal. The composite signal thus has a datarate that is substantially equal to the sum of the data rates of theindividual modulators. An exemplary process of multiplexing two low-ratemodulated sub-signals into a high-rate composite signal is demonstratedin FIG. 2 below.

The composite signal produced by combiner 44 is converted to an analogsignal using a digital-to-analog converter (DAC) 48. The analog signalis upconverted to a suitable radio frequency, filtered and amplified bya transmitter front-end (TX FE) 52, and is transmitted to receiver 28via a transmit antenna 56. At receiver 28, the signal transmitted bytransmitter 24 is received by a receive antenna 60. A receiver front-end(RX FE) 64 amplifies and filters the received signal, and downconvertsit to a suitable intermediate frequency or to baseband. The signal isthen digitized by an analog-to-digital converter (ADC) 68 and thedigitized signal is buffered by a buffer 72.

A stream separator 76 separates the received composite signal into theindividual modulated sub-signals, using methods that are described indetail hereinbelow. The sub-signals are processed in parallel bymultiple demodulators 80, which operate at data rates that are lowerthan the data rate of the composite signal. Each demodulator 80typically demodulates and decodes the FEC of its sub-signal. Thedemodulators may also perform functions such as timing synchronization,carrier recovery, channel equalization and gain control. Typically butnot necessarily, each demodulator 80 processes the sub-signal producedby a particular modulator 36 in transmitter 24.

A multiplexer 84 multiplexes the outputs of demodulators 80 to producean output data stream, which reconstructs the input data stream providedto transmitter 24.

Since modulators 36 and demodulators 80 operate at a reduced data rate,their circuitry can often be simplified and their cost reduced.Additionally, such parallel configuration enables link 20 to operate atdata rates, which may be technologically difficult or impossible toachieve using a single modem. In some cases, the parallel configurationenables link 20 to operate at high data rates while reusing known orexisting lower data rates modem circuitry or devices.

The terms “high data rate” and “low data rate,” which are used herein torespectively describe the data rates of the input data stream and of theindividual modulators and demodulators, should be interpreted asrelative, not absolute terms. In other words, the input data stream canhave any desired data rate (which is the end-to-end data rate of thelink), and the individual modulators and demodulators may have anydesired data rates, as long as the data rate of each individualmodulator and demodulator is lower than the data rate of the input datastream.

Although link 20 is shown in FIG. 1 as a unidirectional link, thisconfiguration is shown purely for the sake of conceptual clarity. Insome embodiments, link 20 comprises a bidirectional link, in which twocommunication systems communicate data to one another. In suchconfigurations, each communication system comprises a transmittersimilar to transmitter 24 and a receiver similar to receiver 28. In somecases, a plurality of known bidirectional modem devices can be used aspairs of modulators 36 and demodulators 80 of a particular communicationstation. For example, Provigent, Inc. (Santa Clara, Calif.) offers asingle-chip modem called PVG-310, which can be used for this purpose.Details regarding this device can be found atwww.provigent.com/products_(—)310.htm.

Modulators 36 and demodulators 80 may all operate at the same data rateand baud rate, or at different rates. Generally, different modulatorsmay use different modulation schemes, FEC codes and data/baud rates.

In some embodiments, demultiplexer 32 and stream combiner 44 coordinatethe order in which the sub-streams are interleaved, so that combiner 44will combine the sub-signals in the correct order. For example, asuitable control interface can be defined between demultiplexer 32 andcombiner 44. In these embodiments, demultiplexer 32 indicates theinterleaving order of the sub-streams to combiner 44 using the controlinterface. This sort of coordination method does not impose overhead orextra bandwidth on the input data stream and sub-streams, but mayrequire control features that are not necessarily supported bymodulators 36.

In alternative embodiments, demultiplexer 32 may insert controlinformation (e.g., known bit sequences) to the sub-streams, so as toindicate to combiner 44 how the sub-signals are to be combined. Thismethod is typically transparent to modulators 36, but reduces theavailable throughput of the link.

Further alternatively, in some cases there is no need for coordinationbetween demultiplexer 32 and combiner 44. For example, in someembodiments is already divided into packets, and demultiplexer 32separates the input data stream into sub-streams in accordance with thispacket structure. Assuming higher layers of the link management areagnostic to the order in which packets are received, there is no needfor coordination between demultiplexer 32 and combiner 44.

FIG. 2 is a diagram that schematically illustrates the conversion of twomodulated sub-signals 88 and 96, which are produced by two respectivemodulators 36, into a high-data-rate modulated composite signal 104, inaccordance with an embodiment of the present invention. Sub-signal 88comprises three frames 92A, 92B and 92C. Sub-signal 96 comprises threeframes 100A, 100B and 100C. The frames in each sub-signal may have thesame length or may differ in length from one another. Similarly, framesof different sub-signals may have the same length or different lengths.In the present example, the two sub-signals have the same data rate.Generally, however, the sub-signals may have different data rates.

Modulated sub-signals 88 and 96 are interleaved by combiner 44 toproduce a composite signal 104, which has a data rate that is twice thedata rate of each sub-signal. Frames 108A, 108C and 108E of signal 104are replicas of frames 92A, 92B and 92C of sub-signal 88 that are playedat a double data rate. Similarly, frames 108B and 108D of signal 104 arereplicas of frames 100A and 100B of sub-signal 96 that are played at adouble data rate (i.e., have a sampling rate that is double the samplingrate of the individual sub-signals).

Since the data rate of composite signal 104 is twice the data rate ofsub-signals 88 and 96, it occupies twice the spectral bandwidth incomparison with each sub-signal. Some considerations related to thespectral mask of the composite signal and the individual sub-signals areaddressed further below.

FIG. 3 is a block diagram that schematically illustrates details ofreceiver 28, in accordance with an embodiment of the present invention.In particular, the figure shows details of stream separator 76 and ofone of demodulators 80. Several exemplary receiver and transmitterconfigurations will be described with reference to FIG. 3. Some of theelements shown in the figure are optional or omitted in someconfigurations.

Modulators 36 and demodulators 80 (collectively referred to as modems)in link 20 may comprise burst modems or stream modems. Burst modemsprocess individual, self-contained bursts of data, which are oftenprocessed irrespective of other bursts. Each burst typically comprises aknown symbol sequence, such as a preamble or midamble, which is used byadaptive loops or other signal processing processes in the demodulatorfor recovering the timing and carrier phase of the burst. The receiverloops often process each burst individually, irrespective ofpreviously-received bursts. In some cases, the receiver loops may useestimated parameters from previous bursts in order to improveperformance. Burst modems are sometimes able to transmit burstscontinuously, so that the transmitted signal appears to be continuous.

Stream modems, on the other hand, process continuous streams of data.Known symbol sequences, such as preambles or midambles, may beperiodically inserted into the symbol stream, but the stream usually hasno definite logical boundaries. The receiver loops of stream modemsoften exploit the continuity of the signal. For example, the carrierrecovery loop of a stream modem is often slow, and assumes that thecarrier phase drifts gradually over time. The transmitter and receiverconfigurations described herein address both burst modems and streammodems.

ADC Sampling Clock Control

Referring to FIG. 3, the sampling clock of ADC 68 in the receiver may befree-running, or it may track the timing of the received signal, asrecovered by one or more of demodulators 80. When the sampling clock ofADC 68 is free-running, a clock oscillator 112 produces the samplingclock and drives ADC 68 via a selector (switch or multiplexer) 120.

Alternatively, the sampling clock of ADC 68 may track, or may be lockedto the transmitter clock, as recovered by one or more of demodulators80. In some embodiments, demodulator 80 comprises a timing estimationunit 136, which estimates the timing offset between demodulator 80 andthe corresponding modulator 36 in transmitter 24, based on the receivedsub-signal. The demodulator further comprises a timing correction unit140, which accepts the offset value, or a desired correction value, fromunit 136 and corrects the demodulator timing accordingly. As a result,the timing of the demodulator tracks the timing of the correspondingmodulator. The exemplary timing recovery loop shown in FIG. 3 has afeed-forward configuration. Alternatively, loops having feedbackconfigurations can also be used.

The timing offset or timing correction estimated by unit 136 cansometimes be used to adjust the sampling clock of ADC 68. In FIG. 3, forexample, the output of unit 136, denoted ‘A’, is provided as a referenceto a phase-locked loop (PLL) 116, which produces the ADC sampling clock.Selector 120 can be set to drive ADC 68 with this clock.

In some embodiments, the timing of the received signal is recovereddigitally. In these embodiments, the timing recovery process involvesboth phase and frequency recovery. When the modems comprise burstmodems, the frequency and the initial phase are typically estimated atthe beginning of each sub-signal interval, and then tracked along theinterval. When the modems comprise stream modems, the frequency andphase are typically tracked in a continuous manner. In some cases,because of the partial duty-cycle operation of each modem, thedemodulator encounters phase discontinuity at the beginning of eachsub-signal interval. Means for compensating for this phase discontinuityare described further below.

In some embodiments, such as when the timing of all modulators 36 intransmitter 24 are synchronized to the same clock, the ADC samplingclock can be controlled by the output of a single timing estimation unit136 in one of demodulators 80. In alternative embodiments, however,different modulators 36 may not be synchronized with one another. Inthese embodiments, the outputs of all timing estimation units 136 ofdemodulators 80 can be used, with each timing estimation unitcontrolling the ADC sampling clock during the interval in which itscorresponding sub-signal is being received.

In some embodiments, once the timing of the received signal isrecovered, timing correction is applied by digitally interpolating, orresampling, the signal at the receiver. The timing offset (timing error)can be estimated either from one or more of the sub-signals at thedemodulators, i.e., at the lower data rates, or from the samples of thecomposite signal, i.e., at the higher data rate. In these embodiments,the timing offset is then corrected using digital interpolation.Interpolation can be applied either to the symbols of the sub-signals,at the lower data rates, or to the samples of the composite signal, atthe higher data rate.

Sub-Signal Boundary Detection Methods

As noted above, the transmitter marks the boundaries between sub-signalsby inserting boundary indicators, and the receiver separates thecomposite signal into the individual sub-signals by automaticallydetecting the boundary indicators.

Separator 76 comprises a demultiplexer 124, which is controlled byselection logic 128. Separator 76 detects the boundaries, ortransitions, between the time intervals used by the different modulatedsub-signals in the composite signal. When the separator detects asub-signal transition (i.e., the end of an interval used by onesub-signal and/or the beginning of another), selection logic 128controls demultiplexer 124, so that the currently-beginning sub-signalis forwarded to the appropriate demodulator 80.

The selection logic can use different mechanisms and criteria fordifferentiating between the different sub-signals interleaved into thereceived composite signal. Some of these mechanisms are self-containedwithin separator 76, while others use synchronization indicationsprovided by demodulators 80.

In some embodiments, transmitter 24 inserts silent periods, whichseparate between adjacent time intervals used by different sub-signals,into the composite signal. Each silent period comprises a time intervalin which the transmitter transmits little or no power. A typical silentperiod duration is on the order of several tens of symbols, although anyother suitable duration can be used. In some cases it is sufficient toreduce the transmitted power level by a given amount, such as by 10-20dB, with respect to the normal average signal power, in order toidentify a time interval as a silent period.

In the context of the present patent application and in the claims, theterm “silent period” is used to describe any time interval in which thesignal power is reduced by a sufficient amount, so as to enable thereceiver to detect this interval and use the detection as a boundaryindicator. Silent periods can be used in conjunction with either burstmodems or stream modems.

In alternative embodiments, the transmitter marks the boundaries betweenthe time intervals used by different sub-signals using sequences ofknown symbols. For example, when using burst modems, each burst istypically preceded by a preamble, which is produced by modulator 36.Inherently, when the different bursts are combined to form the compositesignal, every two adjacent bursts are separated by a preamble. Whenusing stream modems, combiner 44 in transmitter 24 can add appropriateknown symbol sequences at the beginning and/or end of each sub-signalinterval. In some embodiments, the known symbol sequences are added atthe sub-signal level, i.e., at the low data rate. Alternatively,sequences of known samples can be inserted directly into the compositesignals, at the high data rate.

Separator 76 in receiver 28 detects the boundary indicators, i.e., thesilent periods or known symbol sequences, and switches demultiplexer 124accordingly. For example, in some embodiments, separator 76 comprises asample correlator 132, which operates at the high data rate of thecomposite signal.

When the sub-signals are separated by known symbol sequences, correlator132 continuously calculates the correlation between the received samplesof the composite signal and between the desired known sequence. Whencorrelation is detected, e.g., when the output of the correlator crossesa predetermined threshold, the correlator indicates to selection logic128 that a boundary is detected. In response, logic 128 routes thecurrently-beginning sub-signal to the appropriate demodulator byswitching demultiplexer 124.

In some embodiments, each sub-signal (and each modulator/demodulatorpair) is identified by a unique known symbol sequence. In theseembodiments, correlator 132 may indicate the identity of the appropriatedemodulator to selection logic 128. Alternatively, all modems andsub-signals may use the same known sequence. In these embodiments, aswell as when using silent periods, after performing suitable initialsynchronization, the selection logic and multiplexer 124 typicallyalternate among the demodulators in sequential order with each detectedcorrelation.

In some embodiments, although not necessarily, selection logic 128 anddemultiplexer 124 may use a synchronization mechanism for ensuring thatthe appropriate demodulator is switched to demodulate each sub-signal.Such synchronization can be carried out by inserting additional symbolsat the transmitter, which map each sub-signal or sub-stream to theappropriate demodulator.

When using silent periods, an energy or power detector is used insteadof correlator 132. The energy detector calculates the instantaneouspower of the composite signal. When the output of the energy detectorindicates a silent period (e.g., when the output is lower than apredetermined threshold for a predetermined period of time), the energydetector indicates the detection of a boundary to selection logic 128.Any suitable power detection circuit can be used for this purpose.

In some embodiments, some or all of demodulators 80 are able to providesynchronization-related indications, which can be used by separator 76to detect sub-signal boundaries. For example, some demodulators comprisea physical-layer (PHY) correlator 144, which detects correlation with apreamble, midamble or other known symbol sequence used by the modem. Theoutput of the PHY correlator, denoted ‘B’ in FIG. 3, can sometimes beprovided to selection logic 128 and used as a trigger that identifies asub-signal boundary. Although FIG. 3 shows only one PHY correlatoroutput connected to selection logic 128, the PHY correlator outputs ofany or all demodulators 80 can be similarly connected as input to logic128.

In some cases, such as when using stream modems, sequences of knownmodulated symbols can be inserted by multiplexer 32 or by combiner 44 inthe transmitter, in order to mark the boundaries between sub-signals.

In some embodiments, transmitter 24 inserts known bit sequences into theinput data stream in order to mark the boundaries between sub-signalsprocessed by the different modulators and demodulators. For example,when demultiplexer 32 in the transmitter partitions the input datastream into the different sub-streams, it may add a sequence of knownbits at the beginning or end of each sub-stream. These bit sequencesshould not be confused with the known symbol sequences described above,which comprise modulated symbols and not data bits.

Marking sub-stream boundaries (and, consequently, sub-signal boundaries)using known bit sequences can be used, for example, when using streammodems, in which there is no guarantee that each sub-signal begins orends with a known symbol sequence. Alternatively, in some demodulatorsit may not be possible or practical to provide the output of the PHYcorrelator to separator 76. The presence of known bit sequences isusually transparent to the modulator and demodulator. These bitsequences are discarded at the receiver, such as by multiplexer 84.

When the boundaries between sub-signals are marked using known bitsequences, bit correlators 156 at the outputs of demodulators 80calculate the correlation between the demodulated outputs of thedemodulators and the desired bit sequence. The output of the bitcorrelator, denoted ‘C’ in the figure, is provided to selection logic128. The selection logic switches multiplexer 124 based on the detectedbit sequence correlations.

Selection logic 128 may use a single boundary indicator (e.g., theoutput of sample correlator 132, PHY correlator 144 or bit correlator156) or a combination of indicators in order to control multiplexer 124.For example, when using burst modems, the selection logic may combinethe detection of silent periods between bursts, as well as the PHYcorrelator outputs of the modems, in order to determine the sub-signalboundaries. Any suitable priority, criterion or rule can be used byselection logic 128 to determine the boundaries based on the differentindications.

Adaptive Loop Compensation

As noted above, the modulators and demodulators of link 20 are operatedin a partial duty cycle. The demodulators often have adaptive loops thatestimate the timing, phase and/or frequency offset between the modulatorand demodulator, based on the received signal. When modulators 36 anddemodulators 80 comprise stream modulators and demodulators, theseadaptive loops can be disrupted due the partial duty-cycle in which themodems are operated.

The partial duty-cycle effect is usually more significant in streammodems than in burst modems, since burst modems are already operated ina partial duty-cycle in the first place. Nevertheless, when modulators36 and demodulators 80 comprise burst modulators and demodulators, thedemodulators may use timing, phase and/or frequency offset estimatesfrom previous bursts as initial estimates of the loops in acurrently-received burst. These initial estimates can be distorted duethe partial duty-cycle in which the modems are operated.

For example, demodulator 80 comprises a carrier phase estimation unit148, which estimates the carrier phase offset between the demodulatorand the corresponding modulator 36. The output of unit 148 is used by aphase correction unit 152 to compensate for this phase offset. In aconventional stream modem, units 148 and 152 continuously track andcorrect the phase offset over time. When such a modem is operated inlink 20, on the other hand, the modem actually performs modulation anddemodulation, as well as phase tracking and correction, during only partof the time.

The phase error between the modulator and demodulator, however,continues to accumulate over time, regardless of whether the modemactually operates or not. As a result, the demodulator may encounter adiscontinuity in carrier phase at the beginning of each sub-signalinterval, which may degrade its performance or even cause it to lose itsphase lock.

In some embodiments, demodulator 80 and separator 76 estimate andcompensate for the phase that accumulates during the idle period of thedemodulator (i.e., from the end of the previous interval to thebeginning of the current interval of the sub-signal in question), inorder to eliminate or reduce the effect of phase discontinuity. Forexample, demodulator 80 may estimate the accumulated phase, such as bymultiplying the frequency offset between the modulator and demodulator(which is typically calculated by phase estimation unit 148) by theduration of the idle interval. The output of unit 148, denoted ‘D’ inFIG. 3, is provided to a phase correction unit 160 in separator 76,which shifts the phase of the sub-signal to compensate for theaccumulated phase.

In alternative embodiments, when all modulators are synchronized to asingle clock and all demodulators are synchronized to a single clock,phase compensation can be applied to the received composite signal,instead of applying separate phase compensation for each sub-signalusing units 160. Further alternatively, the phase offset of a streammodem can be estimated from a sequence of known symbols or bits at thebeginning of a particular sub-signal interval, and then tracked alongthe interval, similarly to the operation of a burst modem.

In some cases, other adaptive loops of the stream modem, such as timingrecovery or automatic gain control (AGC) may suffer similardiscontinuities over the idle intervals. These discontinuities can becompensated for in a similar manner.

Spectral Mask Considerations

Since the data rate of the composite signal is higher than the data rateof the individual sub-signals, the composite signal transmitted over theair occupies a wider bandwidth in comparison with each sub-signal. Inmany practical cases, however, the spectral response of the compositesignal transmitted by transmitter 24 is required not to exceed a certainspectral mask, such as for satisfying a particular spectrum allocationor for complying with a particular standard or air interface.

When the samples of each sub-signal are multiplexed to form thecomposite signal, the sampling rate of these samples is increased by acertain factor. For example, when three sub-signals sampled at R samplesper second are multiplexed, the sampling rate of the composite signal is3·R. Consequently, the spectral response of the composite signal istypically wider than the spectral response of the individual sub-signalsby the same factor. Thus, designing each sub-signal to meet a spectralmask that is scaled down from the specified mask by this factor usuallyensures that the composite signal meets the specified spectral mask.

Even with properly designed sub-signals, the composite signal maydeviate from the specified spectral mask because of the transientresponse, which is produced when switching from one modulator toanother. Some of this transient response is caused by the truncation ofthe impulse response of a pulse-shaping filter of the modulator.Different measures can be taken in order to reduce or eliminate thesetransients.

For example, when using burst modems, a guard interval, i.e., a silentperiod, can be inserted between successive bursts, in order to allow theimpulse response of the pulse-shaping filter to decay. The length ofthis silent period is on the order of the length of the shaping filter,typically several tens of symbols long. At the beginning of the guardinterval, the demodulator pulse-shaping filter decays from its value atthe end of the previous burst, and the filter ramps up at the end of theguard interval toward the beginning of the next burst.

When using stream modems, the transient response can be reduced bymarking the boundaries between sub-signals using a sequence of knownsymbols, such that the boundary has little or no amplitude/phasediscontinuity. For example, the known symbol sequence can be designed sothat it is symmetrical with respect to its middle and its length is atleast the duration of the transmitter and receiver pulse shapingfilters.

Multiple Modem Communication Method Description

FIG. 4 is a flow chart that schematically illustrates a method forcommunicating at a high data rate using multiple modems, in accordancewith an embodiment of the present invention. The method begins withdemultiplexer 32 in transmitter 24 partitioning the input data streaminto multiple sub-streams and demultiplexing the sub-streams tomodulators 36, at a partitioning step 170. Modulators 36 process therespective sub-streams to produce modulated sub-signals, at a modulationstep 174. Combiner 44 interleaves the sub-signals to form a compositesignal, and the composite signal is transmitted to receiver 28, at aninterleaving and transmission step 178.

Transmitter 24 marks the boundaries between the time intervals used bythe different sub-signals using any of the boundary indicators describedabove, e.g., using silent periods, sequences of known symbols and/orknown bit sequences.

Receiver 28 receives the composite signal, at a reception step 182.Separator 76 identifies the boundary indicators inserted by thetransmitter, at a boundary identification step 186. The separator mayuse any of the identification methods and indications described above,e.g., detection of silent periods and/or detection of a correlation witha know bit or symbol sequence. The separator may use its own detectionmechanisms (such as using an internal sample correlator) or usesynchronization indications provided by demodulators 80.

Based on the identified boundaries, separator 76 demultiplexes thecomposite signal to reproduce the different sub-signals, and distributesthe sub-signals to demodulators 80, at a demultiplexing step 190. Thedemodulators process the respective sub-signals to reproduce thesub-streams, at a demodulation step 194. Multiplexer 84 combines thesub-streams to reconstruct the high-rate data stream.

Although the embodiments described herein mainly address wireless links,the principles of the present invention can also be used in wirelinelinks, as well as in other high data rate communication applications.

It will thus be appreciated that the embodiments described above arecited by way of example, and that the present invention is not limitedto what has been particularly shown and described hereinabove. Rather,the scope of the present invention includes both combinations andsub-combinations of the various features described hereinabove, as wellas variations and modifications thereof which would occur to personsskilled in the art upon reading the foregoing description and which arenot disclosed in the prior art.

1. A method for communication, comprising: receiving a composite signal,which carries data at a first data rate and comprises multiplesub-signals that are interleaved in a time domain and are separated byboundary indicators; demultiplexing the received composite signal intothe sub-signals by automatically detecting the boundary indicatorsbetween the sub-signals in the composite signal; and demodulating thesub-signals using multiple respective demodulators operating at seconddata rates that are lower than the first data rate so as to generaterespective output data streams, and combining the output data streams soas to reconstruct the data, wherein receiving the composite signalcomprises digitally recovering a timing of the composite signal byestimating a timing offset of at least one sub-signal by a respectivedemodulator, and correcting a corresponding timing offset in thecomposite signal using digital interpolation responsively to thecalculated timing offset of the at least one sub-signal.
 2. The methodaccording to claim 1, wherein the composite signal originates from asingle transmitter.
 3. The method according to claim 1, wherein thedemodulators comprise burst demodulators.
 4. The method according toclaim 1, wherein the demodulators comprise stream demodulators.
 5. Themethod according to claim 1, wherein the boundary indicators comprisesilent periods.
 6. The method according to claim 5, whereinautomatically detecting the boundary indicators comprises calculating aninstantaneous power of the received composite signal and detecting theboundary indicators responsively to the calculated instantaneous power.7. The method according to claim 1, wherein the boundary indicatorscomprise sequences of known symbols.
 8. The method according to claim 7,wherein automatically detecting the boundary indicators comprisescalculating correlations between samples of the received compositesignal and between the sequences of the known symbols.
 9. The methodaccording to claim 7, wherein automatically detecting the boundaryindicators comprises accepting one or more correlation indications fromone or more of the multiple demodulators, which indicate a correlationbetween the sub-signals demodulated by the respective demodulators andbetween the sequences of the known symbols.
 10. The method according toclaim 1, wherein the boundary indicators comprise sequences of knownsamples having the first data rate, and wherein automatically detectingthe boundary indicators comprises calculating correlations betweensamples of the received composite signal and between the sequences ofthe known samples.
 11. The method according to claim 1, wherein theboundary indicators comprise sequences of known bits.
 12. The methodaccording to claim 1, wherein demodulating the sub-signals comprisesrecovering a timing of at least one sub-signal by the respectivedemodulator, and wherein receiving the composite signal comprisesdigitizing the composite signal using an analog-to-digital converter(ADC), and adjusting a sampling clock of the ADC based on the recoveredtiming.
 13. The method according to claim 1, wherein the compositesignal comprises a sequence of known samples, and wherein recovering thetiming of the composite signal comprises estimating the timing offset inthe composite signal by processing the known samples.
 14. The methodaccording to claim 1, wherein at least one of the demodulators comprisesan adaptive receiver loop used for demodulating the respectivesub-signal, and wherein demodulating the sub-signals comprisescompensating for an error of the adaptive loop accumulated betweensuccessive time intervals of the sub-signal.
 15. The method according toclaim 14, wherein the adaptive loop comprises a phase recovery loop, andwherein compensating for the error comprises compensating for a phaseerror accumulated between the successive time intervals of thesub-signal.
 16. The method according to claim 14, wherein the adaptiveloop comprises a timing recovery loop, and wherein compensating for theerror comprises compensating for a timing error accumulated between thesuccessive time intervals of the sub-signal.
 17. The method according toclaim 1, wherein at least two of the second data rates are differentfrom one another.
 18. A receiver, comprising: a front-end, which isarranged to receive a composite signal, which carries data at a firstdata rate and comprises multiple sub-signals that are interleaved in atime domain and are separated by boundary indicators; a separator, whichis arranged to demultiplex the received composite signal into thesub-signals by automatically detecting the boundary indicators betweenthe sub-signals in the composite signal; multiple demodulators, whichoperate at second data rates that are lower than the first data rate andare arranged to demodulate the respective sub-signals so as to generaterespective output data streams; and a multiplexer, which is configuredto combine the output data streams so as to reconstruct the data,wherein at least one of the demodulators is arranged to estimate atiming offset of a respective sub-signal, and wherein the receiverfurther comprises a timing adjustment circuit, which is arranged tocorrect a corresponding timing offset in the composite signal usingdigital interpolation, responsively to the calculated timing offset ofthe sub-signal.
 19. The receiver according to claim 18, wherein thecomposite signal originates from a single transmitter, which interleavesthe sub-signals and inserts the boundary indicators.
 20. The receiveraccording to claim 18, wherein the demodulators comprise burstdemodulators.
 21. The receiver according to claim 18, wherein thedemodulators comprise stream demodulators.
 22. The receiver according toclaim 18, wherein the boundary indicators comprise silent periods. 23.The receiver according to claim 22, wherein the separator comprises apower detector, which is arranged to calculate an instantaneous power ofthe received composite signal and to detect the boundary indicatorsresponsively to the calculated instantaneous power.
 24. The receiveraccording to claim 18, wherein the boundary indicators comprisesequences of known symbols.
 25. The receiver according to claim 24,wherein the separator is arranged to calculate correlations betweensamples of the received composite signal and between the sequences ofthe known symbols and to detect the boundary indicators based on thecorrelations.
 26. The receiver according to claim 24, wherein at leastsome of the demodulators comprise correlators, which operate at therespective second data rates calculate correlation indications betweenthe sub-signals demodulated by the respective demodulators and betweenthe sequences of the known symbols, and wherein the separator isarranged to accept the correlation indications from the demodulators inorder to detect the boundaries.
 27. The receiver according to claim 18,wherein the boundary indicators comprise sequences of known sampleshaving the first data rate, and wherein the separator is arranged tocalculate correlations between samples of the received composite signaland between the sequences of the known samples and to detect theboundary indicators based on the correlations.
 28. The receiveraccording to claim 18, wherein the boundary indicators comprisesequences of known bits, and comprising a bit correlator, which iscoupled to an output of one of the demodulators and is arranged tocorrelate the output data stream at the output of the one of thedemodulators with the sequences of known bits so as to detect theboundary indicators.
 29. The receiver according to claim 18, wherein oneof the demodulators is arranged to recover a timing of the sub-signaldemodulated by the demodulator, and comprising an analog-to-digitalconverter (ADC), which is arranged to digitize the composite signal inaccordance with a sampling clock, and a timing adjustment circuit, whichis arranged to adjust the sampling clock of the ADC based on therecovered timing of the sub-signal.
 30. The receiver according to claim18, wherein the composite signal comprises a sequence of samples, andwherein the separator is arranged to estimate the timing offset in thecomposite signal by processing the samples.
 31. The receiver accordingto claim 18, wherein at least one of the demodulators comprises anadaptive receiver loop used for demodulating the respective sub-signal,and comprising an adjustment circuit, which is arranged to compensatefor an error of the adaptive loop accumulated between successive timeintervals of the sub-signal.
 32. The receiver according to claim 31,wherein the adaptive loop comprises a phase recovery loop, and whereinthe adjustment circuit is arranged to compensate for a phase erroraccumulated between the successive time intervals of the sub-signal. 33.The receiver according to claim 31, wherein the adaptive loop comprisesa timing recovery loop, and wherein the adjustment circuit is arrangedto compensate for a timing error accumulated between the successive timeintervals of the sub-signal.
 34. The receiver according to claim 18,wherein at least two of the second data rates are different from oneanother.
 35. A communication link, comprising: a transmitter, whichcomprises multiple modulators and is arranged to demultiplex input datahaving a first data rate into respective sub-streams having second datarates that are lower than the first data rate, to modulate thesub-streams using the respective modulators that operate at the seconddata rates, to interleave the modulated sub-streams in a time-domain toproduce a modulated composite signal, to insert boundary indicatorsbetween the modulated sub-streams in the composite signal and totransmit the composite signal; and a receiver, which comprises multipledemodulators that respectively operate at the second data rates and isarranged to receive the composite signal, to demultiplex the receivedcomposite signal into the sub-signals by automatically detecting theboundary indicators, to respectively demodulate the sub-signals usingthe demodulators so as to generate output data streams, and to combinethe output data streams so as to reconstruct the input data, wherein atleast one of the demodulators is arranged to estimate a timing offset ofa respective sub-signal, and wherein the receiver further comprises atiming adjustment circuit, which is arranged to correct a correspondingtiming offset in the composite signal using digital interpolation,responsively to the calculated timing offset of the sub-signal.
 36. Amethod for communication, comprising: demultiplexing input data having afirst data rate into multiple sub-streams having respective second datarates that are lower than the first data rate; modulating thesub-streams using respective modulators operating at the second datarates; interleaving the modulated sub-streams in a time-domain toproduce a modulated composite signal; inserting boundary indicatorsbetween the modulated sub-streams in the composite signal; transmittingthe composite signal over a communication channel; receiving thecomposite signal transmitted over the communication channel;demultiplexing the received composite signal into the sub-signals byautomatically detecting the boundary indicators; demodulating thesub-signals using respective demodulators operating at the second datarates so as to generate output data streams; and combining the outputdata streams so as to reconstruct the input data, wherein receiving thecomposite signal comprises digitally recovering a timing of thecomposite signal by estimating a timing offset of at least onesub-signal by a respective demodulator, and correcting a correspondingtiming offset in the composite signal using digital interpolationresponsively to the calculated timing offset of the at least onesub-signal.